1. Field of the Invention
This invention relates generally to RF antennas and more particularly to an array of dual trough radiating elements capable of scanning from broadside to endfire.
2. Description of Related Art
Circularly polarized antenna arrays for radiating electromagnetic energy at microwave frequencies are generally known. However, scanning a circular polarized antenna including trough radiating elements is more difficult to achieve than scanning a specific polarization due to the greatly different aspects that each component wave, vertical and horizontal polarization, sees over the scanned volume. For example, even if the elements have the same phase center, i.e., physical location, each polarization""s phase shift to the far field is likely to be vastly different at or near endfire as compared to broadside radiation, the reason being the attenuation of each polarization is different as is shown in FIG. 1. In FIG. 1, a horizontally polarized signal tends to be shorted out as it propagates near a conducting surface, such as a ground plane, while the vertical polarization signal propagates relatively unattenuated. Thus, endfire radiation which comprises RF energy radiated coplanar with the ground plane is severely restricted due to the attenuation of the horizontally polarized signal.
The above-noted attenuation problem near endfire is solved by the subject invention in two ways: (a) by using a trough or notch radiator whose launch point is already a quarter wavelength up from ground, and (b) by utilizing a switchable parasitic ground plane structure in connection with switchable circuit elements that are activated or turned on when a beam to be radiated is scanned to or near endfire while being turned off for broadside radiation.
The present invention in its principal aspect is directed to a circularly polarized trough antenna, which is comprised of: an array of dual trough radiator elements including orthogonal trough waveguide cavities and RF feed members of predetermined adjustable length extending across the cavities from one radiator element to its neighbor, where the feed members are suspended in a slot formed in the body radiator elements and where the inner or proximal end is connectable to an RF energy source while the outer or distal end is unconnected in an open circuit arrangement; intermediate support members of electrical insulation are located on an outer surface of the radiator elements; and a parasitic ground plane structure consisting of a set of parasitic conductor elements are located on a top surface of the intermediate support members so as to enable scanning of the array to or near endfire when energized. In a preferred embodiment, the parasitic conductor elements are connectable to a source of electrical potential by a switching circuit arrangement.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while disclosing the preferred embodiment of the invention, is given by way of illustration only inasmuch as various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.